1. Field of the Invention
The present invention relates to transducers and, more particularly, to a transducer which converts small displacements into light intensity variations.
2. Description of the Prior Art
Underwater target locating systems may utilize active acoustic devices, which comprise a radiation source and a receiver to detect reflected sound energy, or passive devices which detect sound energy radiated from distant sources. The information obtained with active and passive systems is the same; mainly they determine the relative position of a multiplicity of disbursed discrete targets in a quiescent water ambient. Passive underwater locating systems are generally preferred for military applications. Since target position cannot be determined passively with a single acoustic receiver (hydrophone), a passive system requires a multiplicity of hydrophones, the minimum number to determine the position in one plane being three, one at each position at the vertices of an equiangular triangle. These systems determine the relative position of a target by frequency filtering the target signal received at each sensor from the background noise, determining the relative phases of the filtered signal between sensors, and processing this phase information to obtain the relative range and angle location. Large numbers of hydrophones, 100 and 1000, are typically utilized in a one or two dimensional array to provide sufficient systems signal sensitivity in the presence of noise and to provide a desired angular resolution over a broad acoustic frequency band.
Acoustic transducers of the prior art utilize piezoelectric crystals or ferroelectric ceramics to transform acoustic signals into electrical signals by converting pressure variations into corresponding voltage variations across electrodes positioned on opposite sides of the material. These transducers typically supply very small voltages at very high impedance levels. Generally the transducer is coupled to an amplifier via a long wire or coaxial cable, the capacitance of which is charged by the voltage across the output terminals of the crystal, causing the voltage response, due to a given pressure wave, to be greatly reduced. This reduction in sensitivity may be eliminated by positioning a transimpedance amplifier in close proximity to the transducer which amplifies the signal and transforms the high output impedance of the transducer to a low impedance which is coupled to the input terminals of the transmission line. The transimpedance amplifier eliminates the decreased signal caused by the capacitance of the transmission line, thus permitting the amplified signal to be transmitted with little loss over long lengths of time.
Due to the complexity of hydrophone arrays their costs generally are excessive. Moreover, the large number of electronic components, each having finite failure rates, utilized in the array make it near impossible to maintain all parts of such a complicated system in perfect working order for more than a few hours. In view of the deficiencies of the prior art transducer, efforts have been expended to develop electrically passive acoustic transducers utilizing fiber optic techniques. Fiber optic systems eliminate active components at the transducer, provide higher bandwidth, smaller cable diameter, lower weight, and lower cost. Generally the effort expended on fiber optic transducers has been in the area of single mode interferometric devices. These devices, however, require long lived laser sources, single mode, single polarization fibers, and low loss single mode connectors, each of which require advances in the state of the art before practical elements utilizable in a fiber optic transducer system can be developed. Additionally, single mode interferometric systems exhibit relatively high sensitivity to ambient pressure head and temperature variations. Of all the problems that exist in prior art fiber optic sonar transducers, reduction of phase variations caused by the sensitivity of the single mode fiber to the same pressure head and temperature variations, is the most severe. Ambient phase variations produced in the fiber optic cable may be minimized by increasing the length of the fiber at the transducer. This increase, however, creates more severe pressure head and temperature induced phase variations in the transducer for which compensation via electrically active feed back control systems may be required. The limitations of single mode interferometric sensor systems are overcome with the present invention by utilizing multimode fibers and devising an intensity modulation technique that is compatible therewith.